As an example for understanding the present invention, liquefied nitrogen, LN.sub.2, is typical of cryogens, and is widely used in industry and in health care. In many applications, liquid nitrogen is stored in various sized dewars near the point of use, and smaller amounts are withdrawn from the dewar to be utilized in an apparatus or instrument as needed. An example of use in health care is set forth in U.S. Pat. No. 4,116,199. Examples of the use of small amounts of nitrogen in industry are given in U.S. Pat. No. 5,222,999 and U.S. Pat. No. 5,237,836. In these examples, nitrogen may be stored near an industrial workstation or within a doctors' offices or other treatment facility in a dewar ranging from 5 liters to 50 liters, depending upon the particular application involved. In use, the nitrogen is withdrawn from the storage dewar and placed in the container of a utilization device, such as the examples referred to hereinbefore.
Typically, the liquid cryogen is withdrawn from a storage dewar by means of what is known in the art as a withdrawal device. This constitutes a plug which fits into the top of the dewar in place of the storage plug, and is held tightly in place by means of expansion, or by means of hold-down devices, such as springs, straps, or clamps. Within the plug there is provided a common pressure relief valve, which typically is set to maintain the gas pressure in the dewar no greater than about 4 pounds per square inch.
The device has a long tube which extends downwardly through the plug to the bottom of the dewar. It extends outwardly of the plug and turns to become horizontal, where a gas-tight valve is fitted. Outwardly of the valve, the tube may be bent downwardly somewhat to direct the liquid toward a container to be held near the dewar, and the liquid flows out of the tube through a sintered bronze filter, which softens the flow (much like the aerator on a kitchen faucet). When liquid cryogen is to be withdrawn, the valve is opened; when the container (e.g., of the utilization device) is full, the valve is closed once again. The withdrawal unit remains attached to the dewar, immersed in the cryogen at all times except when the dewar is being refilled.
The principal problem with such a withdrawal device is that the presence of the tube within the liquid inside the storage dewar continuously pumps heat from the atmosphere outside into the liquid, causing it to continuously boil, converting liquid to gas, which escapes through the pressure relief valve. A typical rate of evaporation is over 12 grams per hour (816 grams per liter). The typical 10 liter dewar with the withdrawal device known in the art mounted therein has a static holding time (that is, the time when all 10 liters of cryogen will evaporated simply by sitting in the storage dewar, with no liquid being removed) of approximately 27 days. This means, in use, various amounts of the liquid is wasted, depending on how much is actually used. While nitrogen is extremely inexpensive (compared to other industrial substances and medical modalities), the delivery of a small amount to a doctor's office, or the like, can be unnecessarily expensive, due to the evaporation caused by the withdrawal device.
Another, less significant problem is that should an operator become distracted, such as dropping a device being filled, or otherwise, a withdrawal device known to the prior art can rapidly empty an entire storage dewar so long as the valve is left open.
A solution to this problem is offered by U.S. Pat. No. 5,488,831. This invention is a removable withdrawal device comprised of a plug for insertion into the neck of the dewar. A withdrawal tube, with a passive heat device disposed at the bottom, passes through the plug for retrieval of the liquid. The heat device causes the liquid to boil, which creates pressure to force an amount of liquid up through the withdrawal tube and out through a sintered bronze filter. However this device may not work adequately with large dewars or when the dewar is almost empty. It was also found through testing that the amount of liquid removed at one time was dependent on the amount of brass on the end of the withdrawal tube, as well as the amount of liquid in the dewar. Thus the amount of cryogen that would be removed at one time is not always predictable.